The cover of golf balls has been generally formed from ionomer resin (e.g. Japanese Patent Kokoku Publication No. 49-27093). Particularly, in a two-piece solid golf ball using a solid core, the ionomer is exclusively used as the cover. The reason is that the ionomer is superior in durability, cut resistance and rebound characteristics and is easily processed.
The ionomer is generally a copolymer of an olefin monomer and an .alpha.,.beta.-ethylenic carboxylic acid of which a portion of carboxylic acid groups is neutralized with metal ion. However, the ionomer is inferior in shot feel and controllability (ease of spinning) to a balata (transpolyisoprene) cover for a multi-layer structure golf ball having a thread rubber layer because of high hardness and high rigidity.
Softening the ionomer has been considered for improving the shot feel and controllability in this field.
For example, Japanese Patent Kokai Publication Nos. 1-308577 and 5-3931 suggest that a soft ionomer which is prepared by neutralizing a terpolymer of an .alpha.-olefin, an unsaturated carboxylic acid (e.g. acrylic acid, methacrylic acid, etc.) and an acrylate ester with sodium ion or zinc ion high rigid ionomer is blended or mixed with the above mentioned hard type ionomer, which improves the shot feel and controllability of golf balls.
However, even if the above soft ionomer is blended, the shot feel similar to the balata covered golf ball has not been obtained. The blend of the soft ionomer adversely affects on rebound characteristics and flight performance. The blend of the soft ionomer is considered by the art to be insufficient.